Research aircraft HALO examines cirrus clouds and vapour trails

April 2014

Unanswered questions about the formation of clouds and their impact on the climate are currently setting limitations on the validity of global climate forecasts. To make a detailed analysis of the climate effects of natural ice clouds and the vapour trails created by air traffic, the HALO research aircraft (a collective initiative by German environment and climate research bodies) embarked on the first of a total of 12 measurement flights on 24 March 2014. For four weeks, as part of the ML-CIRRUS (Mid-Latitude Cirrus) mission, ice clouds (also known as 'cirrus') will be measured at altitudes of between eight and 14 kilometres above Europe and the North Atlantic. Under the auspices of the DLR Institute of Atmospheric Physics, a team of around 100 scientists from various atmospheric research institutes will be studying the formation, lifecycle, and climate effect of both naturally occurring cirrus and vapour trail cirrus clouds.

The scientists will be paying particular attention to the long-lasting vapour trail cirrus created by air traffic. To date, the extent to which the properties of these anthropogenic clouds differ from natural cirrus remains unknown. New findings indicate that the climate-warming effect of vapour trail cirrus may be greater than that of the carbon dioxide emissions from aircraft. Locally, there are massive differences. With HALO, a comprehensive set of measurement data will be collected, which will allow to build better models and reduce the existing uncertainties.

The atmospheric researchers on board HALO will be supported in their work by a set of innovative instrumentation. To prepare for ML-CIRRUS, new methods have been developed for several years to take very precise measurements of the properties of cirrus clouds and vapour trails. HALO will now be flying with one of the most modern sets of cloud research instrumentation in the world – altogether, the experimental payload weighs just under three tons. Besides the lidar (a laser experiment for remote cloud sensing), the equipment on board this research aircraft includes nine different cloud probes mounted under the wings and a complex set of aerosol, water vapour and trace gas instrumentation in the cabin.

When the weather is clear, thin cirrus clouds and vapour trails can regularly be seen in the sky. They wrap around the Earth like a warming scarf. The actual strength of their warming effect is the subject of current research and depends on many parameters such as the size and number of ice crystals in the clouds, the altitude of these clouds, and their lifespan. The shape of the ice particles also influences their radiation effects. The results will provide the basis for detailed process models, which will help improve global climate forecasts.